This invention relates to a process of producing sponge iron by a direct reduction of iron oxide containing materials with solid carbonaceous reducing agents in a rotary kiln, wherein the material discharged from the rotary kiln is separated into sponge iron and material which contains surplus carbon, the carbon-containing material is burnt in a fluidized bed reactor containing a circulating fluidized bed, and the heat generated by the combustion is dissipated and utilized to produce electric power.
In the production of sponge iron by a direct reduction with solid carbonaceous reducing agents in a rotary kiln, only the fixed carbon (FC) content of the reducing agent can be utilized for the reduction. In most cases the combustible volatile constituents of the reducing agent can be used only in part to preheat the charge in the heating-up zone. As a result, the exhaust gas still contains combustible gaseous constituents as well as entrained dusts, which consist in part of combustible reducing agent. The content of combustible volatile constituents in the exhaust gas is relatively high, particularly if coals having a high content of volatile constituents, such as brown coals, are used. For this reason the exhaust gas must be afterburnt if its combustible heat content is to be recovered in addition to its sensible heat content. In order to avoid a reoxidation of the sponge iron product the process must be so controlled that the material discharged from the kiln contains a relatively large or relatively small surplus of carbon in dependence on the reactivity of the coal used as a reducing agent. In addition to that surplus, the material which is discharged contains the ash which has been formed and the sulfur, combined as CaS, which are separated from the sponge iron.
It is known from Laid-open German Application 33 00 867 to afterburn the exhaust gas from the rotary kiln in an afterburning chamber, to utilize the heat content of the exhaust gas for the production of steam, and to utilize the steam for a generation of electric power. In order to avoid overheating and incrustation, the afterburning must be carried out in a plurality of stages and with cooling by sprayed water for temperature control. The latent heat of evaporation cannot be utilized for the production of steam. In that process the fine-grained solids contained in the exhaust gas are not completely burnt. The nonmagnetic material which has been separated from the sponge iron contained in the material discharged from the rotary kiln is burnt in a circulating fluidized bed. The heat generated by the combustion is used via a steam generator for the generation of electric power. The combustible constituents of the exhaust gas are utilized only in part and the SO.sub.2 which is contained in the exhaust gas and constitutes a pollutant is not removed.